Since the rotational speed of a tire for a high performance motorcycle can be high, the tire will be largely affected by a centrifugal force, thereby expanding the tread portion of the tire outward, which may impair the steering stability. For this reason, a tire structure in which a reinforcing member (spiral member) made of organic fiber or steel is wound on the tread portion of a tire almost in parallel to the tire equatorial plane is being developed. As the reinforcing member spirally wound along the tire equatorial plane, a nylon fiber, an aromatic polyamide (aramid), steel or the like is used. Among others, an aromatic polyamide or steel does not extend even at a high temperature and can minimize the expansion of the tread portion of the tire; therefore the tire does not expand due to the centrifugal force even when the tire revolves at a high speed, whereby steering stability and durability can be improved.
Examples of a tire in which, by arranging a spiral belt layer in such a manner, the stiffness in the circumferential direction is increased, lift deformation at a high speed running is prevented, the traction performance is improved, and as the result the steering stability is further improved, includes a tire according to Patent Document 1.
However, a faster and higher output modern motorcycle had a problem of compatibility between the traction performance and the turn performance. To the center portion of a tread which is frequently used, a higher traction is applied due to a high output, and therefore, the traction performance needs to be secured by making the tread rubber have a high hardness. When a spiral belt layer is arranged or when the tread rubber is made to have a high hardness, however, the grip while turning was not necessarily satisfactory.
In order to solve the above problems, for example, in Patent Document 2, it is disclosed that, by dividing a tread rubber into three parts in the radial direction and replacing the whole rubber of the shoulder portion with the one with a low hardness compared with the rubber at the center portion, the contact length of the shoulder portion when turning at a large camber angle can be sufficiently extended. By this method, a contact area can be sufficiently secured and grip while turning can be improved.